As we enter a new century that will present novel problems of law and warfare, it is good to review some of the crucial concepts regarding precautionary intervention that have not always been clearly defined in the public debate. For example, what are differences between pre-emption, preventative war and preclusive war? And what is the underlying connection between intervention on humanitarian grounds, intervention to prevent the development of nuclear weapons, and intervention to thwart terrorists?

Pre-emption is the use of force in anticipation of an imminent attack. In 1837, Canadian rebels, operating out of a base on Navy Island in Lake Ontario, were receiving arms and men from Buffalo by means of transport on the Caroline, a private schooner. On December 29, a party of British marines crossed into American waters, set the Caroline on fire, and cut her adrift to be carried across Niagara Falls by the tide. Several of her crew were killed - though she ran aground before actually going over the falls. The British ambassador, Henry Fox, justified the attack on the ground of pre-emptive self-defence. In reply, the American secretary of state, Daniel Webster, wrote that a state has a right of pre-emptive self-defence only where the "necessity of that self-defence is instant, overwhelming, and leaving no choice of means, and no moment for deliberation".

As a matter of law, it is certainly true that any right of self-defence implies a right to act while action is still possible. The test for imminence is applied to the harm, not to the overt act sought to be prevented. As one commentator has observed: "If waiting for 'imminence' implies waiting until it is no longer possible to act effectively, such a right would be illusory." The doctrine of pre-emption is well recognised in international law. Pre-emptive war as a doctrine has an ancient lineage. Francis Bacon was not the first political adviser to disdain academic counsel when he wrote: "Neither is the opinion of some of the Schoolmen to be received: that a war cannot justly be made, but upon a precedent injury or provocation. For there is no question, but a just fear of an imminent danger, though no blow be given, is a lawful cause of war."

Preventative war, by contrast, is initiated on the belief that armed conflict, while not imminent, is inevitable and that any postponement of hostilities will work to the disadvantage of the state that is contemplating action. In the 20th century, preventative war is associated with unprovoked aggression. Hitler's occupation of Norway, for example, was justified on the grounds that Norway, if unoccupied, would eventually be invaded by Britain to Germany's disadvantage. The Japanese attack on Pearl Harbour is another notorious example of preventative war. Preventative war is usually considered illicit.

A third concept, preclusive intervention, is often confused with the first two (which are often confused with each other). The lawful grounds for preclusive warfare do not depend on the imminence of a threat to the intervener, which distinguishes these grounds from those of preemption. The Nato countries that authorised action against Serbia over its ethnic cleansing in Kosovo did not claim that they faced the threat of an imminent attack. But neither are the grounds for preclusive intervention entirely up to the intervener, as is the case with preventative war. Rather, there must be a further finding: that the target state has radically compromised its sovereignty, by its own acts; and that, by continuing its offences, it has definitively refused to rectify this compromise. Such compromises occur when a state violates international undertakings to a degree that imperils international security - refusal to comply with UN security council conditions for a ceasefire or providing terrorists with weapons of mass destruction might be examples - or gravely threatens its own people by campaigns of genocide, ethnic cleansing or slavery, all of which have been internationally recognised as delicts.

The important but as yet underdeveloped condition is the recognition by an international body of the compromised sovereignty that renders a state vulnerable to intervention, whether by economic sanctions, threats of military action or armed attack.

Certainly a UN security council resolution under chapter VII of the UN charter is sufficient. Perhaps the unanimous action by a regional security organisation such as Nato or the Organisation of American States is also sufficient, insofar as these have been recognised by the UN charter. But what of the action of a single state, as occurred when Tanzania invaded Uganda to stop the atrocities of the Idi Amin regime, or when Vietnam overthrew the Khmer Rouge - an intervention the UN security council, led by the US, refused to endorse? (Note that these cases are different from interventions, such as those of France in the Comoros or the US in Panama, in which a legitimate regime is overthrown and seeks outside assistance.)

Unless we have a clear international standard to govern these cases, they will never be seen as legitimate by many people. After all, Japan invaded Manchuria with trumped-up claims of self-defence, and Hitler menaced Czechoslovakia with fictitious charges of anti-German persecution. Above all, we will not have the public trust without which preclusive strategies cannot succeed. That is because, of all these three notions, the preclusive most depends upon a willingness to accord governments faith in their assessments of the future.

President Clinton has often remarked that the greatest failure of his two terms in office was the refusal of the US to intervene in Rwanda to stop the killing of Tutsis and moderate Hutus in 1994. Suppose the US had intervened. What would have happened? My own judgment is that the president would have been impeached by the Republican congress that took office in January 1995, but that can only be a surmise. What we can be sure of is that American lives would have been lost in a conflict for which there was no public support, and that innocent Rwandan civilians would inevitably have been killed even in the most successful circumstances. If the president had justified his actions by saying that 800,000 lives had been saved, who would have believed him?

Developing standards for preclusive intervention is an urgent matter because the war aim in the war against terror is not the acquisition of territory or the forced adoption of any ideology or rejection of any religion - it is the protection of civilians. The potential threat to civilians posed by armed groups and states dedicated to mass killing has given new urgency to preclusive measures, including theatre missile defences, strengthened alliances, robust international legal institutions, extended deterrence and regional denuclearisation. It is this threat that links intervention on humanitarian grounds, intervention to prevent the development of nuclear weapons and intervention to deny sanctuaries to terrorists.

· Philip Bobbitt is the author of Terror and Consent: The Wars for the Twenty-first Century comment@guardian.co.uk